The tracking issue for this feature is: #28979

The non_ascii_idents feature adds support for non-ASCII identifiers.

#![allow(unused_variables)]
#![feature(non_ascii_idents)]

fn main() {
const ε: f64 = 0.00001f64;
const Π: f64 = 3.14f64;
}

^Lexer:
IDENTIFIER :
      XID_start XID_continue^*
   | _ XID_continue⁺

An identifier is any nonempty Unicode string of the following form:

Either

• The first character has property XID_start
• The remaining characters have property XID_continue

Or

• The first character is _
• The identifier is more than one character, _ alone is not an identifier
• The remaining characters have property XID_continue

that does not occur in the set of strict keywords.

Note: XID_start and XID_continue as character properties cover the character ranges used to form the more familiar C and Java language-family identifiers.